G protein dependent signaling pathways regulate biological processes triggered by many hormones, inflammatory mediators, neurotransmitters and sensory stimuli, thereby coordinating a diverse array of cell, tissue and organ functions in the adult. These pathways also regulate embryonic development. Perturbations of G protein signaling pathways are associated with many human diseases, including cancer, heart failure, asthma and endocrine disorders. G protein signaling pathways are the targets of more than half of the drugs used today in clinical medicine. Therefore, basic research of G protein signaling pathways will continue to reveal novel disease mechanisms and fuel the discovery process for novel therapeutic agents used to treat a variety of human diseases. The long term goal of this project is to delineate the mechanisms whereby receptors and G proteins function in cell-cell signaling pathways that control the proliferation, differentiation and morphogenesis of eukaryotic cells. Two major challenges are to determine how receptors activate G proteins, and how downstream components of the signaling pathway regulate cell motility and morphogenesis. To answer these questions, this project uses the mating pheromone response pathway of the yeast S. cerevisiae as a model that is directly applicable to cells of many other eukaryotic organisms including humans. Studies of G protein signaling in yeast have revealed novel mechanisms that later have been shown to be used in human cells. The following specific aims are proposed: 1. How do receptors activate G proteins? 2. Does oligomerization regulate receptor assembly, trafficking to the plasma membrane, signaling, or downregulation? 3. How do PAK family kinases regulate actin cytoskeletal organization?